Cigarette Paper Having a High Diffusion Capacity During Thermal Decomposition

ABSTRACT

The invention relates to cigarette paper which contains a water-soluble salt, preferably sodium bicarbonate, potassium bicarbonate or ammonium carbonate, as a result of which a high diffusion capacity during thermal decomposition, and thus a reduction in harmful carbon monoxide in the cigarette smoke, is achieved. The invention relates in particular to cigarette paper comprising at least one water-soluble salt which has lost more than 15% of its initial mass after heating to 230° C. The invention further relates to a cigarette produced from the cigarette paper, to a method for producing the cigarette paper and to the use of the water-soluble salt.

This application is a continuation of Patent Cooperation Treaty Application PCT/EP2011/003743, filed Jul. 26, 2011, which in turn claims priority from German Patent Application 10 2010 032 814.6, filed Jul. 30, 2010; which are incorporated herein by reference.

The present invention relates to a cigarette paper, which contains a water-soluble salt, preferably sodium bicarbonate, potassium bicarbonate or ammonium carbonate, as a result of which a high diffusion capacity during thermal decomposition, and thus a reduction of carbon monoxide in the cigarette smoke, is obtained.

BACKGROUND OF THE INVENTION

It is a matter of customary knowledge that cigarette smoke contains a lot of harmful substances, including carbon monoxide, among others. Therefore, there exists in the industry a great deal of interest in producing cigarettes the smoke from which contains considerably less harmful substances. To reduce the amount of such substances, cigarettes are very often provided with filters, typically made out of cellulose acetate. These filters are, however, not capable of reducing the amount of carbon monoxide in the smoke of the cigarette, since cellulose acetate cannot absorb the carbon monoxide. Various proposals regarding adding catalysts to the filter to convert carbon monoxide into less harmful carbon dioxide were not successful, partly due to functional and partly due to economic reasons.

Diluting the smoke generated in the cigarette is also known, for example with air flowing through the perforations of the tipping paper. This technique has the advantage that it can reduce the concentration of carbon monoxide in the cigarette smoke, but the disadvantage is that it also dilutes the substances which create the taste of the cigarette and hence deteriorates the taste sensation of the cigarette and customer acceptance.

Furthermore, it is known that by increasing the diffusion of gas through the cigarette paper, carbon monoxide can be selectively reduced. Various attempts to increase the diffusion constant of cigarette paper, for example by selecting an appropriate filler material particle distribution, are known in the prior art. Although these attempts enjoyed early successes, they still cannot increase the diffusion constant of the cigarette paper so substantially that a substantial reduction of carbon monoxide can be achieved.

High carbon monoxide values are observed especially for self-extinguishing cigarettes, which are known in the prior art. Such cigarettes use cigarette paper with flame retardant bands on it in order to be classified as self-extinguishing in a standardized test (ASTM E2187-04). This test has been embodied into law, for example in the USA, Canada, Australia and Finland. The increased carbon monoxide values are caused by the fact that carbon monoxide is not able to diffuse out of the cigarette through the flame retardant bands. Therefore, there exists in the industry an interest in providing cigarette papers which compensate for this undesirable side-effect.

Typical cigarette papers comprise cellulose fibres obtained from wood, flax or other materials. In addition, mixtures of cellulose fibres of different origin are used. Cigarette papers have a typical basis weight of 10 g/m² to 60 g/m², whereby the range of 20 g/m² to 35 g/m² is generally preferred.

Cigarette papers often comprise inorganic, mineral filler materials which are added to the paper in a mass fraction of 10% to 40%. A frequently used filler material is chalk (calcium carbonate). However, other oxides and carbonates such as magnesium oxide or aluminum hydroxide are also used.

Cigarette paper can also be provided with burning salts, which increase or reduce the burn rate of the paper. Tri-sodium and tri-potassium citrate and mixtures which are added to the paper in an amount of 0% to 5% of the paper mass, are very frequently used. The group of burning salts of technical relevance additionally further comprises citrates, malates, tartrates, acetates, nitrates, succinates, fumarates, gluconates, glycolates, lactates, oxalates, salicylates, α-hydroxycaprylates and phosphates. The paper is, for example, impregnated with a solution or a suspension of such burning salts in a size press, or the solution or suspension is applied to the surface of the paper in a film press.

A typical property of cigarette paper of high technical relevance is its air permeability. This describes the permeability of the paper for an air flow caused by a pressure difference between the two sides of the paper. It therefore indicates the volume of air flowing through the paper per unit time, per unit area and per pressure difference and it thus has the unit cm³/(min cm² kPa). It is often termed the CORESTA unit (CORESTA Unit, CU), whereby 1 CU=1 cm³/(min cm² kPa). Typical cigarette papers have an air permeability between 10 CU and 300 CU, whereby the range from 20 CU to 120 CU is preferred. The air permeability can, for example, be determined according to ISO 2965.

Another important property of cigarette paper is its diffusion capacity. The diffusion capacity is a transfer coefficient and describes the permeability of the cigarette paper to a gas flow caused by a concentration difference. It therefore indicates the volume of gas flowing through the paper per unit time, per unit area and per concentration difference and hence has the unit cm³/(cm² s)=cm/s. The diffusion capacity of a cigarette paper for CO₂ can be measured with a CO₂ Diffusivity Meter from the company Sodim, for example. Typical cigarette papers have a diffusion capacity between 0.1 cm/s and 3.5 cm/s at room temperature; the range from 0.5 cm/s to 3.0 cm/s is preferred.

The diffusion capacity can be measured at room temperature or under standard conditions, for example at a temperature of 23° C. and a relative air humidity of 50%, after conditioning the paper appropriately. Alternatively, it is also possible to determine the diffusion capacity of the paper after the paper has been exposed to thermal stress, in particular by elevated temperatures.

Both the air permeability and the diffusion capacity are determined by the pore structure of the cigarette paper, and so there is a relationship between these properties. It is technically difficult to adjust the diffusion capacity of the cigarette paper independently of the air permeability of the paper in the paper making process. In particular, in most cases the air permeability is part of the specifications set by the cigarette manufacturers; this means that in practice, the diffusion capacity results from the paper making process and can only be varied within a small range. Hence, there exists a particular interest in finding papers with a diffusion capacity which increases only on demand, that is, precisely at the time when the temperature of the paper is increased by the glowing tip of the smoldering cigarette.

The substances in cigarette smoke are determined using a method in which the cigarette is smoked under standardized conditions. An example of such a method is described in ISO 4387. In this method, the cigarette is initially lit at the beginning of the first puff and then every minute a puff is taken at the mouth end of the cigarette which lasts 2 seconds, has a volume of 35 cm³ and has a sinusoidal profile. The puffs are repeated on the cigarette until the length of the cigarette drops below a length defined in the standard. The smoke flowing out of the mouth end of the cigarette during the puffs is collected in a Cambridge Filter Pad and afterwards this filter is chemically analyzed having regard to the various substances it contains, for example nicotine. The gas phase flowing out of the mouth end of the cigarette during the puffs and through the Cambridge filter pad is collected and also chemically analyzed, for example to determine the quantity of carbon monoxide in the cigarette smoke.

During the standardized smoking process, the cigarette is thus in two different flow states. During the puff, there is a noticeable pressure difference, typically in the range from 200 Pa to 1500 Pa between the inner side of the cigarette paper facing the tobacco and the outside of the cigarette paper. This pressure difference causes air to flow through the cigarette paper into the tobacco section of the cigarette and dilutes the smoke generated during the puff. During this phase, which lasts for 2 seconds per puff, the extent of the dilution is determined by the air permeability of the paper.

However, in the period between puffs, the cigarette smolders without a considerable pressure difference between the inside of the tobacco section of the cigarette and the environment, so that the gas transport is determined by the gas concentration difference between the tobacco section and the environment. This means that carbon monoxide can diffuse through the cigarette paper out of the tobacco section into the ambient air, particularly from the region of the glowing tip. In this phase, which lasts for 58 seconds per puff, the diffusion capacity is the parameter responsible for the reduction of the carbon monoxide.

To reduce the amount of carbon monoxide in the smoke, it is thus important for the diffusion capacity of the cigarette paper to be high, in particular in the area of the glowing tip, as the carbon monoxide is generated there. It is thus of a particular advantage for the diffusion capacity to be high or to increase rapidly once the cigarette paper is exposed to elevated temperatures by the approach of the glowing tip.

When smoking a cigarette, a so called “char line” can be clearly identified in the region of the glowing tip, which separates the practically completely thermally destroyed cigarette paper from the still substantially intact cigarette paper. Measurements in the prior art show that the so called “char line” has a temperature of about 450° C. Correspondingly, it is necessary for the diffusion capacity of the cigarette paper to be high or to increase strongly at temperatures considerably below 450° C.

One objective of the invention is to provide a cigarette with an increased diffusion capacity which allows a reduction of carbon monoxide in the cigarette smoke.

SUMMARY OF THE INVENTION

The invention achieves this objective by impregnating or coating the cigarette paper with one or more water-soluble salts which degrade at a comparatively low temperature, i.e. at considerably less than 450° C., and thereby open up the pore structure of the paper on a smoldering cigarette manufactured from this paper to allow significantly better diffusion of carbon monoxide out of the cigarette.

In particular, the objective of the present invention is achieved by means of a cigarette paper which comprises at least one water-soluble salt which after heating up to 230° C. loses more than 15% of its initial mass, whereby the heating, starting at a temperature of 30° C., takes place at a heating rate of 5° C./min at a nitrogen flow of 25 ml/min.

In one embodiment, the water-soluble salt loses more than 20%, preferably more than 25%, particularly preferably more than 30% and most preferably more than 35% of its initial mass.

In one embodiment, the water-soluble salt is an inorganic salt or a mixture of inorganic salts.

In one embodiment, the water-soluble salt is a bicarbonate, preferably an alkali metal bicarbonate or an ammonium bicarbonate, or a carbonate, preferably an ammonium carbonate. Mixtures of these salts are also contemplated.

In a preferred embodiment, the water-soluble salt or the alkali metal bicarbonate respectively is a sodium- or potassium bicarbonate. Mixtures of both of these salts are also contemplated.

In a particularly preferred embodiment, the water-soluble salt or the alkali metal bicarbonate respectively is a potassium bicarbonate.

In one embodiment, the quantity of the water-soluble salt is 0.1% to 10%, preferably 1% to 6%, particularly preferably 3% to 6% of the paper mass. These quantities can also be obtained by a mixture of different water-soluble salts.

In a preferred embodiment, the water-soluble salt is sodium bicarbonate in a quantity of at least 4%, preferably 4% to 10%, particularly preferably 4% to 6% of the paper mass.

In an alternative preferred embodiment, the water-soluble salt is potassium bicarbonate, with at least 3%, preferably 3% to 10%, particularly preferably 3% to 6% of the paper mass.

In a further alternative preferred embodiment, the water-soluble salt is ammonium carbonate with at least 4%, preferably 4% to 10%, particularly preferably 4% to 6% of the paper mass.

In one embodiment, the water-soluble salt is contained in areas of the cigarette paper, preferably in discrete stripe-shaped areas.

In a preferred embodiment, the areas are designed such they form one or more bands in the circumferential direction of the tobacco rod on a cigarette manufactured from this cigarette paper.

This can be advantageous, for example, if the cigarette paper known in the prior art has already been treated with diffusion capacity reducing, flame retardant stripes such that a cigarette manufactured therefrom would be classified as self-extinguishing in a standardized test (ASTM E2187-04). Typically, these flame retardant stripes are positioned so that they form bands in the circumferential direction on the cigarette. Such cigarettes exhibit higher carbon monoxide values because less carbon monoxide can diffuse through the flame retardant stripes out of the cigarette. To compensate for this effect, it can be useful to apply the salts according to this invention to the as yet untreated areas of the cigarette paper, that is, in the areas between the flame retardant stripes, so that a compensating higher diffusion capacity is obtained in these areas during thermal decomposition of the cigarette paper.

In a particularly preferred embodiment, the areas are separated from the flame retardant areas, preferably the flame retardant stripes. The term “separated” means that the areas are separated from the flame retardant areas, but also a certain overlap can be accommodated which, for example, might be caused by the production process.

In one embodiment, the cigarette paper is impregnated with the water-soluble salt (soaked).

In an alternative embodiment, the cigarette paper is coated on one or on both sides with the water-soluble salt.

Furthermore, the objective of the invention is achieved by means of a cigarette paper which comprises at least one water-soluble salt, wherein the at least one water-soluble salt is a bicarbonate, preferably an alkali metal bicarbonate or an ammonium bicarbonate, or a carbonate, preferably an ammonium carbonate. Mixtures of these salts are also contemplated.

In a preferred embodiment, the alkali metal bicarbonate is a sodium- or potassium bicarbonate. Mixtures of both of these salts are also contemplated.

In a particularly preferred embodiment, the alkali metal bicarbonate is a potassium bicarbonate.

In one embodiment, the quantity of the water-soluble salt defined above is 0.1% to 10%, preferably 1% to 6%, particularly preferred 3% to 6% of the paper mass.

In a preferred embodiment, the water-soluble salt defined above is a sodium bicarbonate with a content of at least 4%, preferably 4% to 10%, particularly preferably 4% to 6% of the paper mass.

In an alternative preferred embodiment, the water-soluble salt defined above is a potassium bicarbonate with at least 3%, preferably 3% to 10%, particularly preferably 3% to 6% of the paper mass.

In a further alternative preferred embodiment, the water-soluble salt defined above is an ammonium carbonate with at least 4%, preferably 4% to 10%, particularly preferably 4% to 6% of the paper mass.

In one embodiment, the water-soluble salt defined above is contained in discrete areas of the cigarette paper, preferably in stripe-shaped areas.

In a preferred embodiment, the areas with the water-soluble salt defined above are designed such that they form one or more bands in the circumferential direction of the tobacco rod on a cigarette manufactured from this paper.

In a particularly preferred embodiment, the areas are separated from the flame retardant areas.

In one embodiment, the cigarette paper is impregnated (soaked) with the water-soluble salt defined above.

In an alternative embodiment, the cigarette paper is coated on one or both sides with the water-soluble salt defined above.

The objective of the invention is further achieved by means of a cigarette which comprises a cigarette paper according to the invention.

The cigarette can be manufactured on customary cigarette making machines using the cigarette paper with the aid of further, in some cases optional components like tobacco, tipping paper, filter, plug wrap paper and adhesive.

The object of the invention is further achieved by means of a process for producing a cigarette paper according to this invention, wherein the process comprises the following steps:

-   -   (1) providing a cigarette paper or a fibre-filler suspension         with less than 50%, preferably less than 30% water content,         based on the total mass of the paper or the total mass of the         fibre-filler suspension respectively.     -   (2) applying an aqueous solution with at least one water-soluble         salt, whereby the quantity of the water-soluble salt in the         solution is 0.2% to 20%, preferably 2% to 15%, based on the         total mass of the solution.

In one embodiment of the process, the water-soluble salt is an inorganic salt. Mixtures of inorganic salts in the aqueous solution are also contemplated.

In one embodiment of the process, the water-soluble salt in the aqueous solution is a bicarbonate, preferably an alkali metal bicarbonate or an ammonium bicarbonate or a carbonate, preferably an ammonium carbonate. Mixtures of these salts in the aqueous solution are also contemplated.

In a preferred embodiment of the process, the water-soluble salt or the alkali metal bicarbonate is a sodium or potassium bicarbonate. Mixtures of the two salts in the aqueous solution are also contemplated.

In a particularly preferred embodiment, the water-soluble salt or the alkali metal bicarbonate respectively is a potassium bicarbonate.

In one embodiment, the quantity of the at least one water-soluble salt is 0.1% to 10%, preferably 1% to 6%, particularly preferably 3% to 6% of the paper mass of a cigarette paper manufactured by this process.

In one embodiment, in step (2) of the process, the water-soluble salt is applied in areas of the cigarette paper, preferably in stripe-shaped areas.

In a preferred embodiment, the areas are designed such that they form one or more bands in the circumferential direction of a tobacco rod on a cigarette prepared from this paper.

In a specially preferred embodiment, the areas are separated from flame retardant areas.

In one embodiment, in step (2) of the process, the aqueous solution is applied using a size press or a film press.

In the size press or film press of a paper machine, the aqueous solution is applied to the entire surface on one or both sides of the paper. The paper is therefore essentially impregnated with the solution.

In an alternative embodiment, in step (2) of the process, the aqueous solution is applied using a printing or spraying technique.

Applying the aqueous solution by means of a printing process or by spraying in or after the paper machine allows for the possibility of applying the solution more superficially and only on one side of the paper. This may be advantageous if the solution alters the optical properties of the paper. Then the solution is preferably applied to the side of the paper which later faces the tobacco.

Other processes for applying an aqueous solution to the paper are also contemplated. It is important, however, that the solution is applied at a time during or after the paper production when the cigarette paper already has the ability to absorb the solution and to substantially fix the water-soluble salt in the fibre structure. This will be the case when the paper or the fibre-filler suspension passing through the paper machine respectively has a water content of less than 50% based on the total paper mass, preferably of less than 30% based on the total paper mass, which is in the case with customary paper machines, when the paper has left the press section. It is, for example, not advantageous to add water-soluble carbonates or bicarbonates such as a filler to the fibre-filler suspension before this suspension reaches the paper machine, as the carbonates and bicarbonates present in the solution are lost to a large extent in the dewatering process in the wire section of the paper machine.

The objective of the invention is further achieved by the use of at least one water-soluble salt, preferably an inorganic salt, for increasing the diffusion capacity of a cigarette paper or areas of a cigarette paper during thermal decomposition by more than 0.9 cm/s and/or by more than 50% based on the initial value of the diffusion capacity at 23° C. before the thermal decomposition.

In one embodiment of the use, the water-soluble salt is a bicarbonate, preferably an alkali metal bicarbonate or an ammonium bicarbonate or a carbonate, preferably an ammonium carbonate.

In one embodiment of the use, the water-soluble salt is an alkali metal bicarbonate.

In a preferred embodiment of the use, the alkali metal bicarbonate is a potassium bicarbonate.

In a specially preferred embodiment, the areas are separated by flame retardant areas.

The features of the invention disclosed here and in the patent claims can be essential to the implementation of the invention in its various embodiments individually as well as in any arbitrary combination.

The invention is based on the surprising discovery that a cigarette paper with an increased diffusion capacity can be obtained if the paper is impregnated or coated with specific salts, shown here by sodium bicarbonate, potassium bicarbonate and ammonium carbonate, which decompose at comparatively low temperatures, that is at considerably less than 450° C., and hence open up the pore structure of the cigarette paper. It turns out that the increase in the diffusion capacity of the cigarette paper during thermal decomposition can in some cases be increased more than two-fold. Such an increase of the increase of the diffusion capacity can contribute considerably to the reduction of harmful carbon monoxide in the smoke from the cigarette manufactured from this paper.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the weight loss of water-soluble salts during heating in a thermo gravimetric analysis. The loss of mass as a % of the initial mass is shown in relation to the temperature. Heating to a temperature of up to 600° C., whereby the initial temperature was 30° C., was carried out under a nitrogen gas flow of 25 ml/min. TKZ, tri-potassium citrate; TNZ, tri-sodium citrate.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in detail with reference to the examples and the attached FIGURE.

EXAMPLES Example 1 Production of Cigarette Paper with Sodium Bicarbonate, Potassium Bicarbonate or Ammonium Carbonate

A cigarette paper made out of wood pulp, whereby 32% of the paper mass consists of chalk as filler material, and with a basis weight of 25 g/m², an air permeability of 30 CU and a diffusion capacity of 1.75 cm/s was used for the experiments. This is a customary cigarette paper, so that comparable results can also be expected with other cigarette papers; basically, there are no restrictions in the selection of the cigarette paper, for example as regards air permeability, diffusion capacity, fibre and filler composition or basis weight.

This cigarette paper was impregnated with an aqueous solution of sodium bicarbonate (NaHCO₃), potassium bicarbonate (KHCO₃) or ammonium carbonate ((NH₄)₂CO₃) in the size press using different concentrations.

The same cigarette paper which had been impregnated in the size press with an aqueous solution of tri-sodium citrate (TNZ) or a 50:50 mixture, based on the mass, of tri-sodium citrate and tri-potassium citrate (TNZ/TKZ) respectively, was used for comparison.

Example 2 Measurement of the Diffusion Capacity of the Cigarette Paper Produced

The diffusion capacity of the cigarette papers manufactured in example 1 was determined with a CO₂ Diffusivity Meter from the company Sodim.

The diffusion capacity of the papers at 23° C. and 50% humidity was consistently about 1.75 cm/s. After exposing the papers to a temperature of 230° C. for 30 minutes, they were conditioned at 23° C. and 50% humidity and then the diffusion capacity was measured. The absolute measured values are given in table 1.

TABLE 1 Diffusion capacity in cm/s after heating (230° C., 30 min) Content, as % of paper mass* TNZ TNZ/TKZ NaHCO₃ KHCO₃ (NH₄)₂CO₃ 1 2.168 2.180 2.310 2.285 2.305 2 2.357 2.319 2.472 2.524 2.481 3 2.237 2.362 2.547 2.677 2.574 4 2.367 2.367 2.632 2.838 2.696 5 2.380 2.519 2.666 2.952 2.736 6 2.364 2.568 2.730 3.009 2.824 7 2.365 2.616 2.796 3.112 2.897 8 2.358 2.505 2.739 3.289 2.926 *corresponds to the % by weight

Table 1 shows that the diffusion capacity of papers with sodium bicarbonate, potassium bicarbonate or ammonium carbonate is higher for every concentration in the paper mass than that of papers comprising tri-sodium citrate or tri-sodium/tri-potassium citrate. The highest diffusion capacity measured with citrates is 2.616 cm/s, with 7% tri-sodium/tri-potassium citrate. Higher values than 2.616 cm/s are obtained with sodium bicarbonate and ammonium carbonate in a quantity of 4% and with potassium bicarbonate even in a quantity of 3%. A diffusion capacity of 2.7 cm/s and more could only be obtained with sodium bicarbonate (≧6%), potassium bicarbonate (≧4%) or ammonium carbonate (≧5%), but not with the citrates.

The increase in the diffusion capacity in cm/s compared with the initial value at room temperature of 1.75 cm/s and the relative increase as a percentage based on this initial value are shown in table 2.

TABLE 2 Change in the diffusion capacity in cm/s and in % after heating (230° C., 30 min). Content, as % of the paper mass* TNZ TNZ/TKZ NaHCO₃ KHCO₃ (NH₄)₂CO₃ 1 +0.42 +0.43 +0.56 +0.53 +0.55 (23.9%) (24.6%) (32.0%) (30.5%) (31.7%) 2 +0.61 +0.57 +0.72 +0.77 +0.73 (34.7%) (32.5%) (41.2%) (44.2%) (41.7%) 3 +0.49 +0.61 +0.80 +0.93 +0.82 (27.8%) (35.0%) (45.5%) (53.0%) (47.1%) 4 +0.62 +0.62 +0.88 +1.09 +0.95 (35.2%) (35.3%) (50.4%) (62.1%) (54.0%) 5 +0.63 +0.77 +0.92 +1.20 +0.99 (36.0%) (43.9%) (52.3%) (68.7%) (56.3%) 6 +0.61 +0.82 +0.98 +1.26 +1.07 (35.1%) (46.7%) (56.0%) (71.9%) (61.37%) 7 +0.62 +0.87 +1.04 +1.36 +1.14 (35.2%) (49.5%) (59.8%) (77.8%) (65.5%) 8 +0.61 +0.75 +0.99 +1.54 +1.17 (35.1%) (43.2%) (56.5%) (87.9%) (67.2%) *corresponds to the % by weight

Table 2 clearly shows that through the use of sodium- or potassium bicarbonate or of ammonium carbonate, the increase in the diffusion capacity of the cigarette paper during thermal decomposition can in some cases be increased more than two-fold, compared with citrates as customary burning salts.

The examples additionally show that an increase in the diffusion capacity by more than 0.9 cm/s, starting from an initial value at 23° C. of 1.75 cm/s in this case or a percentage increase in the diffusion capacity of more than 50%, likewise with respect to the initial value at 23° C., is not possible with citrates as customary burning salts, but is possible with the inorganic salt employed. To this end, at least 4% sodium bicarbonate or at least 3% potassium bicarbonate or at least 4% ammonium carbonate must be used.

The examples also show that an increase in the quantity of sodium bicarbonate beyond 6% achieves no further improvement, so that a content of at most 6% based on the paper mass is preferred. However, with potassium bicarbonate an additional increase of the content beyond 6% achieves a further noticeable improvement, so that an upper limit for the reasonable use of potassium bicarbonate cannot be set on the basis of the experiments; instead, the content should be limited to about 10% for reasons of the taste sensation of a cigarette manufactured from this paper. For ammonium carbonate, the achievable improvement for a content in excess of 6% seems to decrease, but does not come to a halt up to a content of 8%, so that here again, a useful upper limit can be drawn at about 10%. Altogether, potassium bicarbonate proves to be superior to sodium bicarbonate or ammonium carbonate as regards the inventive effect, and so its use is particularly preferred.

The cause of the observed effect is probably based on the fact that sodium and potassium bicarbonate decompose at lower temperatures than customary burning salts, namely at about 50° C. Even ammonium bicarbonate and ammonium carbonate decompose at temperatures of about 60° C. and hence at substantially lower temperatures than tri-sodium or tri-potassium citrate, which both only start to decompose from 150° C. A thermo gravimetric analysis of the sodium- and potassium bicarbonates used for application to the paper in table 1, whereby these substances were heated under a nitrogen flow of 25 ml/min at a rate of 5° C./min, from 30° C. to 600° C., indicates that the rapid loss of mass at comparatively low temperatures is responsible for the increase in the diffusion capacity. The results are shown in FIG. 1. It appears that when a temperature of 230° C. is reached, the loss of mass in the case of sodium bicarbonate is about 35% of its initial mass and in the case of potassium bicarbonate, it is about 30% of its initial mass. The FIGURE further indicates that sodium- and potassium bicarbonate not only lead to an increased diffusion capacity, as shown in table 1, but also that it is higher than for customary burning salts in the range from about 130° C. to 230° C.

Thus, it can be assumed that the observed effect may be expected if, under the conditions of this analysis method, the selected inorganic salt has lost 30% to 35%, presumably even smaller fractions, for example from 15% to 25%, of its initial mass at a temperature of 230° C. 

We claim:
 1. A cigarette paper for wrapping the tobacco rod of a cigarette, which cigarette paper comprises at least one water-soluble salt, wherein the water-soluble salt is a salt which after heating to 230° C. has lost more than 15% of its initial mass when the heating, starting at a temperature of 30° C., is carried out at a heating rate of 5° C./min under a nitrogen flow of 25 ml/min.
 2. The cigarette paper of claim 1, wherein said water-soluble salt is a salt which after heating to 230° C. has lost more than 25% of its initial mass when the heating, starting at a temperature of 30° C., is carried out at a heating rate of 5° C./min under a nitrogen flow of 25 ml/min.
 3. The cigarette paper according to claim 1, wherein the water-soluble salt is a bicarbonate.
 4. The cigarette paper of claim 3, wherein the water-soluble salt is a salt selected from a group of an alkali metal bicarbonate, an ammonium bicarbonate, and an ammonium carbonate.
 5. The cigarette paper according to claim 4, wherein the alkali metal bicarbonate is one of a sodium or a potassium bicarbonate.
 6. The cigarette paper according to claim 1, wherein the quantity of the water-soluble salt is 0.1% to 10% of the paper mass.
 7. The cigarette paper according to claim 1, wherein the quantity of the water-soluble salt is 1% to 6%, of the paper mass.
 8. The cigarette paper according to claim 1, wherein the quantity of the water-soluble salt is 3% to 6% of the paper mass.
 9. The cigarette paper according to claim 1, wherein the water-soluble salt is present in discrete areas.
 10. The cigarette paper according to claim 9, wherein said discrete areas are stripe-shaped areas.
 11. The cigarette paper according to claim 10, wherein the areas are designed such that they form one or more bands in the circumferential direction of a tobacco rod of a cigarette manufactured from the cigarette paper.
 12. The cigarette paper according to one of claim 1, wherein the cigarette paper is impregnated with the at least one water-soluble salt.
 13. The cigarette paper according to one of claim 1, wherein the cigarette paper is coated on one or both sides with the at least one water-soluble salt.
 14. A cigarette comprising the cigarette paper, said paper including at least one water-soluble bicarbonate.
 15. A process for manufacturing a cigarette paper, which method comprises the following steps: (1) providing a cigarette paper or a fibre-filler suspension with less than 50% water content with respect to the total paper mass or the total mass of the fibre-filler suspension respectively, and (2) applying an aqueous solution with at least one water-soluble salt to the cigarette paper or the fibre-filler suspension passing through the paper machine respectively, whereby the quantity of water-soluble salt in the solution is 0.2% to 20% with respect to the mass of the solution, wherein said water-soluble salt is a salt which after heating to 230° C. has lost more than 15% of its initial mass when the heating, starting at a temperature of 30° C., is carried out at a heating rate of 5° C./min under a nitrogen flow of 25 ml/min.
 16. The process of claim 15, wherein the fibre-filler suspension has a water content of less than 30% with respect to the total paper mass or the total mass of the fibre-filler suspension respectively.
 17. The process of claim 15, wherein the quantity of water-soluble salt in the solution is 2% to 15%, with respect to the mass of the solution.
 18. The process according to claim 15, wherein the water-soluble salt is a bicarbonate.
 19. The process according to claim 18, wherein the water-soluble is a salt selected from a group of an alkali metal bicarbonate, an ammonium bicarbonate, and an ammonium carbonate.
 20. The process according to claim 19, wherein the alkali metal bicarbonate is one of a sodium- or a potassium bicarbonate.
 21. The process according to claim 15, wherein the aqueous solution is applied using a size press or a film press.
 22. The process according to claim 15, wherein the aqueous solution is applied using a printing or spraying technique.
 23. Use of at least one water-soluble salt to increase the diffusion capacity of a cigarette paper, or of areas of a cigarette paper for wrapping the tobacco rod of a cigarette, during thermal decomposition by more than 0.9 cm/s or by more than 50% based on the initial value of the diffusion capacity at 23° C. before the thermal degradation.
 24. Use according to claim 23, wherein the water-soluble salt is a bicarbonate.
 25. Use according to claim 24, wherein the water soluble salt is a salt selected from a group of an alkali metal bicarbonate, an ammonium bicarbonate, and an ammonium carbonate.
 26. Use according to claim 25, wherein the alkali metal bicarbonate is one of a sodium or a potassium bicarbonate.
 27. A cigarette paper for wrapping the tobacco rod of a cigarette, said cigarette paper comprising at least one water-soluble salt, wherein the water-soluble salt is a bicarbonate.
 28. A cigarette paper according to claim 27, wherein said water-soluble salt is a salt selected from a group of an alkali metal bicarbonate, an ammonium bicarbonate, and an ammonium carbonate.
 29. A cigarette paper according to claim 28, wherein said alkali metal bicarbonate is one of a sodium or a potassium bicarbonate. 